Generally, a steel wire which is used for a sawing wire or a steel cord which is used as a reinforcement material for radial tires of automobiles, a variety of belts and hoses, or the like, is manufactured by subjecting a wire rod with a diameter of 5-6 mm, which has been hot-rolled and subjected to controlled-cooling, to a primary wire drawing so as to have a diameter of 3-4 mm, and subjecting the wire rod to a patenting treatment and a secondary wire drawing so as to have a diameter of 1-2 mm, and then subjecting the wire rod to a final patenting treatment, brass plating, and a final wet wire drawing process so as to have a diameter of 0.15-0.40 mm.
A steel cord is manufactured by twisting together a plurality of the ultrafine steel wires which obtained in the above manner in a twisting process so as to produce a twisted steel wire.
In general, if a wire is broken when processing a wire rod into a steel wire or twisting a steel wire, productivity and yield rate are significantly degraded. Therefore, there is a strong demand for wire rods or steel wires belonging to the above technical fields to not be broken during the wire drawing process or the twisting process. Among wire drawing processes, in the case of the final wet wire drawing process, since the diameter of a steel wire to be treated is extremely small, the steel wire is highly likely to be broken. Furthermore, in recent years, there has been a trend towards reducing the weight of steel cords or the like for a variety of purposes. As a result, there is a demand for a high strength in the variety of products described above.
In addition, a steel wire used as a PC steel wire, a PC twisted wire, a rope, a PWS wire for bridges, or the like is generally formed into a strand shape by subjecting a wire rod with a diameter of 5-16 mm, which has been subjected to hot rolling and then controlled cooling, to a wire drawing process so as to have a diameter of 2-8 mm, subjecting the rod to molten zinc plating after the wire drawing or in the middle of the wire drawing, according to necessity, and then stranding the rods with or without twisting them together.
Generally, if a wire is broken when processing a wire rod into a steel wire or longitudinal cracks (delamination) occur when twisting the steel wire, productivity and yield rate are significantly degraded. Therefore, there is a strong demand for wire rods or steel wires belonging to the above technical fields to not break during a wire drawing process or a stranding process.
With regard to such products, there was a demand in the past to secure a strength of 1600 MPa or higher as well as to secure sufficient performance in terms of toughness and ductility evaluated by a twisting test or the like, but, in recent years, there has been a trend in which the weight of wires has been reduced for a variety of purposes.
As a result, there is a demand for high strength in a variety of the above products, but it has become impossible to obtain the desired high strength in carbon steel wire rods with a C content of less than 0.9 mass %. Therefore, there has been an increasing demand for steel wires with a C content of 0.9 mass % or higher. However, if the amount of C is increased, since wire drawing properties or torsional properties (delamination resistance) are degraded due to generation of pro-eutectoid cementite (hereinafter, sometimes referred to as ‘pro-eutectoid θ’), wires break more often. As a result, wire rods not only including high amount of C for obtaining high strength but also having excellent wire drawing properties are strongly demanded.
With respect to such recent demands from industries, manufacturing technologies of high carbon wire rods with an amount of C exceeding 1% have been suggested.
For example, Patent Document 1 discloses “a wire rod for high strength and high toughness ultrafine steel wires, a high strength and high toughness ultrafine steel wire, a twisted product using the ultrafine steel wire, and a manufacturing method of the ultrafine steel wire” made of a steel material having a specific chemical composition, in which the average area ratio containing pro-eutectoid cementite is defined. However, since the wire rod suggested in the publication includes one or both of Ni and Co, which are high-priced elements, as essential components, the manufacturing costs are increased.
Patent Document 2 suggests a technology in which 0.6% or more of Al is added so as to suppress generation of pro-eutectoid cementite in a high carbon steel with a content of C exceeding 1%. However, since Al is a strong deoxidizing element, and the amount of hard inclusions that act as a cause of wire breakage during wire drawing is increased, it is difficult to apply the technology to wire rods for steel wires with a small diameter, such as steel cords.
On the other hand, Patent Document 3 suggests a technology in which a high carbon wire rod is heated to an austenite temperature zone, cooled to a temperature range of 823-1023 K, subjected to a deforming process with a degree of 15-80% in the above temperature zone, and then isothermally transformed in a temperature zone of 823-923 K so as to suppress pro-eutectoid cementite. However, since a large facility investment is required to perform a predetermined process in such a temperature zone, there is concern of an increase in manufacturing costs.